Keywords
Kisspeptin - neurokinin B - reproductive disorders
Kisspeptin and neurokinin B (NKB) are hypothalamic neuropeptides that are obligate
for reproductive health. In 2003, two seminal publications reported that inactivating
mutations in the gene encoding for the kisspeptin receptor (previously known as GPR54)
cause hypogonadotrophic hypogonadism and a failure of pubertal development.[1]
[2] Subsequently, it was found that an inactivating mutation of the KISS1 gene also results in normosmic hypogonadotrophic hypogonadism.[3] Conversely, activating mutations of the kisspeptin receptor result in central precocious
puberty.[4] Thus, kisspeptin was shown to play a key role in orchestrating reproductive hormonal
secretion and in the regulation of normal puberty.
Similarly, mutations in genes encoding for NKB or its receptor (TAC3R) also result
in congenital hypogonadotrophic hypogonadism (CHH). The decapeptide NKB is encoded
for by the TAC3 gene and activates the neurokinin 3 receptor (NK3R) encoded by the
TAC3R gene.[5]
[6]
[7]
[8]
[9] Patients with NK3R mutations have reduced gonadotrophin-releasing hormone (GnRH)
pulsatility with an increase in follicle-stimulating hormone (FSH) to luteinizing
hormone (LH) ratio.[10] Patients with either TAC3 or TAC3R mutations were shown to remain responsive to
exogenous kisspeptin administration, consistent with NKB exerting its action upstream
of kisspeptin.[10]
[11]
Since then, it has been established that kisspeptin acts to stimulate hypothalamic
GnRH neurones and thus the remainder of the reproductive axis. In rodent models, kisspeptin
was found to co-localize with NKB and dynorphin to form “KNDy” neurones. These neuropeptides
are thought to act in a paracrine manner to result in the pulsatile secretion of GnRH.
Both central and peripheral administration of kisspeptin in animal models results
in robust stimulation of gonadotrophin secretion.[12]
[13]
Kisspeptin in Healthy Men and Women
Kisspeptin in Healthy Men and Women
In 2005, Dhillo and colleagues conducted the first administration of kisspeptin to
healthy men and observed a dose-dependent increase in gonadotrophin secretion.[14] Thereafter, the same group administered kisspeptin to healthy women; while significant
increases in gonadotrophins were observed in the follicular and luteal phases, LH-rise
was most prominent during the preovulatory phase of the menstrual cycle.[15] Similarly, Chan et al observed that women were least sensitive to an intravenous
bolus of kisspeptin-10 during the follicular phase of the menstrual cycle.[16]
A key feature of a healthy reproductive axis is the pulsatile secretion of GnRH [by proxy serum LH]. A single subcutaneous bolus of kisspeptin was shown to increase LH pulsatility
in healthy women during the follicular phase.[17] Furthermore, a single bolus of kisspeptin-10 could reset the GnRH pulse generator
in men,[18] but this did not appear to be the case in women.[16]
Acute LH responses to intravenous bolus administration of kisspeptin-10 were investigated
in healthy men (dose range: 0.01–3 μg/kg) and the highest LH rises were observed after
1 μg (0.77 nmol/kg).[19] Moreover, a continuous intravenous infusion of 4 μg/kg per hour (3.07 nmol/kg per
hour) of kisspeptin-10 persistently stimulated LH secretion over 22.5 hours.[19] Intriguingly, the LH response to an acute intravenous bolus of kisspeptin-10 (0.3 μg/kg)
was similar in obese hypogonadal diabetic men to that in healthy control subjects.[20] Furthermore, robust LH stimulation was observed to an intravenous infusion of kisspeptin-10
(4 μg/kg per hour) in obese hypogonadal diabetic men for 11 hours.[20] Additionally, responses to kisspeptin in healthy older men were also maintained.[21] Thus, in future, kisspeptin-based therapeutics could be developed for the treatment
of functional hypogonadism such as diabetes, obesity, or age-related hypogonadism.
Kisspeptin in Hypothalamic Amenorrhea
Kisspeptin in Hypothalamic Amenorrhea
Hypothalamic amenorrhea (HA) is a condition characterized by a reduction in the physiological
pulsatile secretion of GnRH. HA frequently occurs in the context of low body weight,
excessive exercise, reduced energy availability, psychological stress, or genetic
predisposition.[22]
Notably, the acute LH response to kisspeptin was increased by four-fold in women with
HA compared with the same dose in healthy women during the follicular phase.[23] Data from a rodent model of HA could explain this observation; Kiss1 expression in the hypothalamus is reduced in undernourished rodents, whereas kisspeptin
receptor expression is increased.[24] Thus, HA could be considered as a state of hypothalamic kisspeptin deficiency, and
thus there has been great interest in investigating the use of kisspeptin to restore
physiological hormonal secretion in these women. Twice daily subcutaneous administration
of kisspeptin-54 (6.4 nmol/kg) for 2 weeks was investigated in five women with HA.[23] While robust increases in serum gonadotrophin levels were observed on day 1 of administration,
these were reduced within a few days of administration,[25] and markedly attenuated by day 14.[23] The majority of the tachyphylaxis to kisspeptin is likely to have occurred at the
level of the kisspeptin receptor as GnRH responsiveness was maintained.[23] However, twice weekly administration of subcutaneous kisspeptin was able to persistently
stimulate gonadotrophin secretion.[25] Thus, whilst bolus administration of kisspeptin has potential to restore normal
reproductive function in women with HA, there is a risk of tachyphylaxis with excessive
doses of kisspeptin. An alternative approach is to use a continuous intravenous infusion
of kisspeptin, which when administered at lower doses can restore physiological LH
pulsatility without tachyphylaxis.[26] Ongoing studies are assessing whether continuous subcutaneous administration of
kisspeptin could be used to restore ovulatory function in women with HA.
Kisspeptin During In Vitro Fertilization
Kisspeptin During In Vitro Fertilization
Kisspeptin signaling is requisite for physiological ovulation; infusion of a kisspeptin
neutralizing antibody directly into the preoptic area of the hypothalamus results
in abolition of the midcycle LH surge.[27] Furthermore, kisspeptin administration to superovulated prepubertal rats was able
to induce ovulation to a similar extent as human chorionic gonadotrophin (hCG).[12] Additionally, the LH response to kisspeptin is dramatically increased in the preovulatory
phase of the menstrual cycle, suggesting that it could induce an ovulatory LH surge.[15]
Indeed in 2014, Jayasena and colleagues conducted a “proof of concept” study demonstrating
that a single subcutaneous bolus of kisspeptin-54 (1.6–12.8 nmol/kg) was able to induce
an LH surge with a peak of approximately 40 IU/L that lasted for 12 to 14 hours.[28] This was sufficient to induce oocyte maturation in 51 of 53 healthy subfertile women
undergoing in vitro fertilization (IVF) treatment.[28] Furthermore, kisspeptin safely induced high rates of oocyte maturation in 60 women
at increased risk of ovarian hyperstimulation syndrome (OHSS) based on elevated ovarian
reserve markers without causing this most serious of complications of current IVF
treatment.[29] OHSS is predominantly caused by hCG stimulating the release of vascular endothelial
growth factor (VEGF) from the ovary, which increases vascular permeability and leakage
of fluid into the third spaces of the body, resulting in ascites, pleural effusions,
renal impairment, and rarely even death.[30] Kisspeptin reduced the odds of severe OHSS by 33.6-fold (95% CI: 12.6–89.5) compared
with hCG.[31] More recently, kisspeptin has been hypothesized to play a direct role in the pathogenesis
of OHSS, by directly reducing estradiol-induced VEGF production.[32] This is consistent with clinical data, suggesting that extending the duration of
the LH surge with a second dose of kisspeptin further enhances oocyte maturation,
but without causing OHSS.[33]
Kisspeptin is known to be present in the ovary and has been suggested to play a direct
ovarian role in addition to its predominant mode of action through hypothalamic GnRH
secretion. Ovarian kisspeptin expression is undetectable in immature oocytes, but
is increased at ovulation.[34] Kisspeptin has been shown to increase in vitro maturation of ovine[35] and porcine immature oocytes,[36] and smaller follicles contributed more to the oocyte yield than following other
triggers in humans.[37] Furthermore, intrafollicular kisspeptin levels correlate with the number of mature
oocytes retrieved.[5]
Owens and colleagues[38] examined the in vivo and in vitro actions of using kisspeptin-54 trigger on gene expression relating to ovarian reproductive
function, steroidogenesis, and OHSS in granulosa lutein cells, compared with current
triggers.[38] They observed that triggering oocyte maturation with kisspeptin-54 increased the
expression of genes involved in ovarian steroidogenesis such as LH/hCG and FSH receptor,
steroid acute regulatory (STAR) protein, aromatase, and estrogen receptors.[38]
In summary, kisspeptin stimulates release of an LH surge sufficient to induce effective
oocyte maturation and achieve at least comparable pregnancy rates in subfertile women
undergoing IVF treatment. Crucially, rates of OHSS are dramatically reduced by kisspeptin
triggering, and this could in part be achieved by a further direct ovarian action
of kisspeptin on VEGF production. Randomized controlled trials directly comparing
kisspeptin to current triggers of oocyte maturation with accurate determination of
clinical outcomes, such as OHSS rates, are now indicated.
Kisspeptin as a Diagnostic Test of Hypothalamic Function
Kisspeptin as a Diagnostic Test of Hypothalamic Function
Kisspeptin acts to stimulate the hypothalamus to secrete GnRH and thus has potential
as a diagnostic test of hypothalamic reproductive function, where currently no direct
test exists. The hypothalamus is known to play a key role in controlling the onset
of puberty; mutations in several genes involved in the regulation of hypothalamic
GnRH function are known to result in CHH and absent puberty. Responses to kisspeptin
in CHH have been shown to be attenuated[39]; however, patients with reversal of CHH (which can occur in up to one-fifth of such
patients) regain responsiveness to kisspeptin.[40] The majority of patients with delayed puberty have constitutional delay of growth
and puberty (CDGP) and will proceed through puberty with time; however, a subset of
patients will have CHH and are unlikely to proceed through puberty without treatment.
Thus, kisspeptin has been evaluated as a diagnostic test in children with delayed
puberty and a wide variety of responses to a kisspeptin challenge test were observed.[41] Follow-up of puberty onset in these children will reveal whether those who responded
to kisspeptin were more likely to have CDGP than CHH.
In summary, mutations affecting structural components of hypothalamic GnRH signaling
(e.g., anosmin1) are unlikely to respond to kisspeptin (unless reversal has occurred)[40]; however, functional hypogonadotrophic hypogonadism (e.g., HA[23] or hyperprolactinemia[42]) does respond to kisspeptin. Thus, a “kisspeptin test” can be used to more accurately
evaluate the hypothalamic contribution to hypogonadotrophic hypogonadism.
NKB Administration in Humans
NKB Administration in Humans
NKB administration in male agonadal juvenile monkeys increases LH levels.[43] However, infusions of NKB were unable to stimulate gonadotrophin secretion or pulsatility
in either healthy men or women.[44] Nevertheless, an interesting phenomenon was reported by some participants who had
received higher doses, in that they felt hot and appeared flushed.[44] This was investigated in more detail in 10 healthy women who received a 30-minute
infusion of either NKB or vehicle in random crossover design.[45] Sweating, heart rate, and skin temperature were increased during NKB administration,
which are characteristic features of menopausal flushing.[45] Following this, NK3R antagonists have been developed as a novel therapy for menopausal
flushing, reducing the total number and severity of flushes by 41 to 45%.[46] Thus, these promising agents could revolutionize the treatment of women with hot
flushes, especially those at risk of side effects from sex-steroid–based therapies,
and several compounds are now in late-phase development.
NK3 Receptor Antagonism in Polycystic Ovary Syndrome
NK3 Receptor Antagonism in Polycystic Ovary Syndrome
Polycystic ovary syndrome (PCOS) is a condition characterized by anovulation, hyperandrogenism,
and polycystic ovarian morphology.[47] Increased GnRH pulsatility is implicated in the pathogenesis of PCOS and is often
reflected by an increase in serum LH level, which is estimated to be present in half
of women with PCOS. Thus, kisspeptin and NKB pathways could be targeted to treat women
with PCOS. George and colleagues investigated the use of pharmacological blockade
of NKB action to reduce GnRH pulsatility, and assessed the impact of this on clinical
features of PCOS such as hyperandrogenism.[48] Women were randomized to receive an oral NK3R antagonist (AZD4901 at a dose of 20,
40, or 80 mg/day) or placebo for 28 days.[48] After 7 days of 80 mg/day, a 52% reduction in area under the curve (AUC) of LH levels
and a reduction in LH pulsatility by 3.6 LH pulses per 8 hours (p < 0.05) was observed.[48] This was associated with a reduction in total testosterone levels by 28.7%.[48] A secondary analysis revealed that this reduction in androgen levels predominantly
occurred in anovulatory women.[48] Further longer-term studies are indicated to evaluate the efficacy of this novel
approach in the treatment of ovulatory dysfunction in women with PCOS.
Conclusion
Kisspeptin is a critical regulator of hypothalamic GnRH function, and thus offers
a significant opportunity to better evaluate and treat conditions caused by hypothalamic
dysfunction. Several kisspeptin analogs are in development,[49]
[50]
[51]
[52]
[53]
[54] which can aid in the translation of kisspeptin-based therapeutics to the bedside.
Medications targeting NKB signaling are likely to represent a huge advance in the
management of women with postmenopausal hot flushes and could have benefit in other
conditions such as PCOS.
